The invention pertains to a sputter target for cathodic atomization to produce transparent, electrically conductive layers, the target being produced from partially reduced indium oxide-tin oxide powder mixtures or partially reduced co-precipitated indium oxide-tin oxide powders, and to a process for the production of these targets.
Oxide ceramic targets of indium oxide-tin oxide (ITO) are used for the production of thin, transparent, electrically conductive layers by cathodic atomization (sputtering). These layers are used in particular in flat screen imaging technology. The thin indium oxide-tin oxide layers can be produced either by the sputtering of metal targets in a reactive oxygen-containing atmosphere or by the sputtering of oxide ceramic targets.
The sputtering process with oxide ceramic targets has the advantage that the oxygen flow rate in the sputtering chamber is slow, and this makes it easier to control the sputtering process than when sputtering is carried out with metal targets, which requires a high oxygen flow rate.
Because indium oxide/tin oxide is a ceramic material, a critical problem in fabricating the targets is how to make the appropriate powders into sputtering targets which are free of cracks and have sufficient mechanical strength. Otherwise, there is the danger that the target plates will crack apart during final processing or that areas will chip off at the edges. The thermal loads during sputtering operations can also lead to cracks in the target if the ITO material is not strong enough or has insufficient resistance to alternating temperatures.
Another demand to be met consists in that it is necessary to produce targets of high density, because the results of sputtering processes have shown that denser targets offer advantages with respect to the sputtering rate and have less of a tendency to develop black spots. The latter advantage is especially important, because it guarantees trouble-free, continuous operation of the target and of the sputtering system.
Partially reduced indium oxide-tin oxide targets and their production are described in U.S. Pat. No. 4,647,548. According to this publication, indium oxide-tin oxide powder mixtures are hot-pressed under reducing conditions at 850.degree.-1,000.degree. C.; the oxides can be hot-pressed either in a graphite hot-press mold or in a mold to which carbon or carbon-releasing organic material has been added. During pressing, the oxides are partially reduced, so that a target is formed which contains less oxygen than that called for by the stoichiometry of the composition.
By means of this process, however, the only targets which can be produced are those with densities of &lt;91% of the theoretical density (TD), poor electrical resistivity (.rho.= 0.1-0.6 .OMEGA.Pcm), and insufficient mechanical strength. All three factors have negative effects on the use of the sputtering target.
A process for the production of partially reduced ITO targets by the use of a pre-reduced ITO powder followed by hot-pressing is described in DE 41 24 471. The task of this prior process was to produce ITO targets with a degree of reduction which varied by less than 5% by compacting highly homogeneous, reduced powder by hot-pressing to form target disks.
Studies have shown, however, that, unless the morphology of the partially reduced powder is favorable, very poor results will be obtained with respect to target density and the mechanical strength of the target disks.